Short Excited-state Lifetime Research Articles

Triplet Photosensitizers Showing Strong Absorption of Visible Light and Long-Lived Triplet Excited States and Application in Photocatalysis: A Mini Review

Triplet photosensitizers (PSs) have been studied as photocatalysts in photocatalytic H2 evolution by water splitting and photoredox catalytic synthetic organic reactions, etc. The applications share common features that the photocatalysts (triplet PSs) harvest the photoexcitation energy, then undergo intersystem crossing (ISC), and finally initiate the electron transfer or triplet energy transfer, which are intermolecular processes in most cases. Thus, triplet PSs showing a strong visible-light-harvesting ability and long triplet excited state lifetimes are desired because they can enhance the above intermolecular processes. The conventional transition metal complexes (i.e., Ru(bpy)3X2 or Ir(ppy)3) have been widely used; however, these complexes are not satisfactory due to their weak absorption of visible light and short triplet excited state lifetimes. To a large extent, the photophysical property of these complexes is due to the low-lying metal-to-ligand charge transfer (MLCT) state, the S0 → 1MLCT transition is weakly allowed, and a strong heavy atom effect exists for the 3MLCT state, which results in weak visible light absorption and a short triplet excited state lifetime, respectively. This mini review introduces the recent progress of the development of the transition metal complexes as well as the organic triplet PSs that show a strong visible-light-harvesting ability and long triplet excited state lifetimes. The application of these new triplet PSs in photocatalytic H2 evolution and photoredox catalytic synthetic organic reactions is summarized.

Short Excited-State Lifetimes Mediate Charge-Recombination Losses in Organic Solar Cell Blends with Low Charge-Transfer Driving Force.

A blend of a low-optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force for electron transfer, is investigated. Using femtosecond transient absorption and electroabsorption spectroscopy, the charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified. Electron transfer is ultrafast, consistent with a Marcus-Levich-Jortner description. However, significant charge recombination and unusually short excited (S1 ) and CT state lifetimes (≈14ps) are observed. At low S1 -CT offset, a short S1 lifetime mediates charge recombination because: i) back-transfer from the CT to the S1 state followed by S1 recombination occurs and ii) additional S1 -CT hybridization decreases the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, relatively slow (tens of picoseconds) dissociation of charges from the CT state is observed, due to low local charge mobility. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield can be increased from the observed 12% to 60% by increasing the S1 and CT lifetimes to 150ps. Alternatively, decreasing the interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.

Efficient Blue Light Emitting Diodes Based On Europium Halide Perovskites.

Flat panel displays enjoy 100 billion-dollar markets with significant penetration in daily life, which require efficient, color-saturated blue, green, and red light-emitting diodes (LEDs). The recently emerged halide perovskites have demonstrated low-cost and outstanding performance for potential LED applications. However, the performance of blue perovskite LEDs (PeLEDs) lags far behind red and green cousins, particularly for color coordinates approaching (0.131, 0.046) that fulfill the Rec. 2020 specification for blue emitters. Here, a high-efficiency, lead-free perovskite, CsEuBr3 , is reported that exhibits bright blue exciton emission centered at 448nm with a color coordinates of (0.15, 0.04), contributed from Eu-5d→Eu-4f/Br-4p transition with an optical band gap of 2.85eV. Further optical characterizations reveal its short excited-state lifetime of 151ns, excellent exciton diffusion diffusivity of 0.0227 cm2 s-1 , and high quantum yield of ≈69%. Inspired by these findings, deep-blue PeLEDs based on all-vacuum processing methods, which have been demonstrated as the most successful approach for the organic LED industry, are constructed. The devices show a maximum external quantum efficiency of 6.5% with an operating half-lifetime of 50 mins at an initial brightness of 15.9cd m-2 . It is anticipated that this work will inspire further research on lanthanide-based perovskites for next-generation LED applications.

Impact of surface heterogeneity on IR line profiles of adsorbed carbon monoxide on models of interstellar grain surfaces

ABSTRACTSurface heterogeneity of model amorphous silica films used as a model for interstellar grain surfaces is revealed through the application of the pre-exponential optimized inversion method to previously reported sub-monolayer thermal desorption studies of carbon monoxide (CO) desorption. The impact of that surface heterogeneity, as represented by the coverage dependence of the CO activation energy for desorption from the amorphous silica surface, on the IR spectroscopy of the CO stretching vibration is explored through vibrational line profile synthesis. Comparison is then made to previous investigations of CO line profiles on this surface and on amorphous solid water as reported in Taj et al. (2017, 2019a). A tentative conclusion is drawn that CO vibrationally promoted desorption from, and diffusion on, the amorphous silica surface may be responsible for the correspondingly short vibrational excited state lifetime of CO on that surface. The contrast with CO on amorphous solid water, where direct and rapid vibrational relaxation into the solid water phonon bath occurs, is highlighted. The consequences of this from the standpoint of CO deposition on grain surfaces are discussed.

Superradiance of bacteriochlorophyll c aggregates in chlorosomes of green photosynthetic bacteria

Chlorosomes are the main light-harvesting complexes of green photosynthetic bacteria that are adapted to a phototrophic life at low-light conditions. They contain a large number of bacteriochlorophyll c, d, or e molecules organized in self-assembling aggregates. Tight packing of the pigments results in strong excitonic interactions between the monomers, which leads to a redshift of the absorption spectra and excitation delocalization. Due to the large amount of disorder present in chlorosomes, the extent of delocalization is limited and further decreases in time after excitation. In this work we address the question whether the excitonic interactions between the bacteriochlorophyll c molecules are strong enough to maintain some extent of delocalization even after exciton relaxation. That would manifest itself by collective spontaneous emission, so-called superradiance. We show that despite a very low fluorescence quantum yield and short excited state lifetime, both caused by the aggregation, chlorosomes indeed exhibit superradiance. The emission occurs from states delocalized over at least two molecules. In other words, the dipole strength of the emissive states is larger than for a bacteriochlorophyll c monomer. This represents an important functional mechanism increasing the probability of excitation energy transfer that is vital at low-light conditions. Similar behaviour was observed also in one type of artificial aggregates, and this may be beneficial for their potential use in artificial photosynthesis.

Tubular Carbon Nitride with Hierarchical Network: Localized Charge Carrier Generation and Reduced Charge Recombination for High‐Performance Photocatalysis of H2 and H2O2 Production

Polymeric carbon nitride (PCN), as an appealing metal‐free and low‐cost photocatalyst for solar‐to‐fuel conversion, is still suffering from insufficient light‐harvesting efficiency and short excited state lifetime of electron–hole pairs. Herein, a 3D hierarchical network of carbon nitride tubes (NCNT) with intercrossing few‐layer tubes of uniform diameter, synthesized by a synergistic strategy of tailoring the chemical and hydrogen bond, is reported. This hollow network structure facilitates the internal light reflection, thus promoting the light‐induced electron–hole pair generation. Moreover, the few‐layer NCNT with nitrogen defects effectively decreases the undesired electron–hole recombination. These advantages are disclosed by experimental spectra and finite difference time domain (FDTD) simulations and density functional theory (DFT) calculations. Impressively, the optimized NCNT exhibits almost 32 times the photocatalytic hydrogen evolution ability (8.6 mmol h−1 g−1) and 4 times (485.7 μmol h−1 g−1) the hydrogen peroxide production of those of bulk PCN, respectively. This work provides a new strategy to construct a hierarchical nanostructure of a high‐performance carbon nitride photocatalyst through enhancing the solar‐light capture and conversion.

Synthesis, Photophysical, and Computational Studies of a Bridged IrΙΙΙ-PtΙΙ Heterodimetallic Complex

An IrIII-PtII heterodimetallic complex [(ppy)2Ir(dapz)PtCl2]Cl (4), together with the corresponding monometallic complexes [(dapz)PtCl2] (2) and [(ppy)2Ir(dapz)]Cl (3) was designed and prepared, where dapz is 2,5-di(N-methyl-N′-(pyrid-2-yl)amino)pyrazine and ppy is 2-phenylpyridine, respectively. Single-crystal X-ray analysis was carried out for complex 4, displaying the intermolecular Pt∙∙∙Pt and aromatic plane∙∙∙plane distances of 3.839 and 3.886 Å, respectively. The monometallic complex 2 exhibits a single emission maximum at 432 nm with a shorter excited-state lifetime (τ) of 6 ns, while complex 3 exhibits an emission band at 454 nm with a longer excited-state lifetime of 135 ns in CH3CN (N2-saturated) under ambient conditions. In contrast, the heterodimetallic complex 4 displays intriguing excitation wavelength-dependent dual singlet and triplet emissions. Theoretical calculations of the electronic structures and absorption spectra of these complexes were carried out to assist the interpretation of these experimental findings.

Excited-State Engineering in Heteroleptic Ionic Iridium(III) Complexes.

ConspectusIridium(III) complexes have assumed a prominent role in the areas of photochemistry and photophysics due to the peculiar properties of both the metal itself and the ligand environment that can be assembled around it. Ir(III) is larger, heavier, and bears a higher ionic charge than its analogue and widely used d6 ions such as Fe(II) and Ru(II). Accordingly, its complexes exhibit wider ligand-field d–d orbital splitting with electronic levels centered on the metal, typically nonemissive and photodissociative, not playing a relevant role in excited-state deactivations. In other words, iridium complexes are typically more stable and/or more emissive than Fe(II) and Ru(II) systems. Additionally, the particularly strong heavy-atom effect of iridium promotes singlet–triplet transitions, with characteristic absorption features in the UV–vis and relatively short excited-state lifetimes of emissive triplet levels. Ir(III) is also a platform for anchoring ligands of rather different sorts. Its versatile chemistry includes not only coordination with classic N∧N neutral ligands but also the binding of negatively charged chelators, typically having a cyclometalating C∧N anchor. The carbon–metal bond in these systems has some degree of covalent character, but this does not preclude a localized description of the excited states of the related complexes, which can be designated as metal-centered (MC), ligand-centered (LC), or charge transfer (CT), allowing a simplified description of electronic and photophysical properties. The possibility of binding different types of ligands and making heteroleptic complexes is a formidable tool for finely tuning the nature and energy of the lowest electronic excited state of cationic Ir(III) complexes by ligand design. Herein we give an account of our work on several families of iridium complexes typically equipped with two cyclometalating bidentate ligands (C∧N), in combination with mono or bidentate “ancillary” ligands with N∧N, C∧N, and C∧C motifs. We have explored new synthesis routes for both cyclometalating and ancillary ligands, obtaining primarily cationic complexes but also some neutral or even negatively charged systems. In the domain of the ancillary ligands, we have explored isocyanides, carbenes, mesoionic triazolylidenes, and bis-tetrazolic ligands. For the cyclometalating moiety, we have investigated carbene, mesoionic triazolylidene, and tetrazolic systems. Key results of our work include new strategies to modify both cyclometalating and ancillary ligands by relocating ionic charges, the determination of new factors affecting the stability of complexes, a demonstration of subtle structural effects that strongly modify the photophysical properties, new options to get blue-greenish emitters for optoelectronic devices, and a set of ligand modifications allowing the optimization of electrochemical and excited-state properties to obtain new promising Ir(III) complexes for photoredox catalysis. These results constitute a step forward in the preparation of custom iridium-based materials crafted by excited-state engineering, which is achieved through the concerted effort of computational and synthetic chemistry along with electrochemistry and photochemistry.

Ultrafast excited-state dynamics of promising nucleobase ancestor 2,4,6-triaminopyrimidine.

The ultrafast excited-state dynamics of 2,4,6-triaminopyrimidine - thought to be a promising candidate for a proto-RNA nucleobase - have been investigated via static multireference quantum-chemical calculations and mixed-quantum-classical/trajectory surface-hopping dynamics with a focus on the lowest-lying electronic states of the singlet manifold and with a view towards understanding the UV(C)/UV(B) photostability of the molecule. Ultrafast internal conversion channels have been identified that connect the lowest-lying ππ* electronically-excited state of 2,4,6-triaminopyrimidine with the ground electronic state, and non-radiative decay has been observed to take place on the picosecond timescale via a ππ* out-of-plane NH2 ("oop-NH2") minimum-energy crossing point. The short excited-state lifetime is competitive with the excited-state lifetimes of the canonical pyrimidine nucleobases, affirming the promise of 2,4,6-triaminopyrimidine as an ancestor. Evidence for energy-dependent excited-state dynamics is presented, and the open question of intersystem crossing is discussed speculatively.

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

Green‐Blue Phosphorescent Iridium(III) Complexes with Near Unitary Quantum Yield

AbstractIridium(III) complexes with cyclometalating ligands can achieve excellent properties for use as phosphorescent emitters in organic light emitting diodes. A series of complexes with 2‐phenyl‐pyridine and bis(pyrazolyl)borate ligands was investigated with regard to this application. The sky‐blue to green emitters showed nearly unitary quantum yields and short excited state lifetimes.

Carbazole‐Based Iridium(III) Complexes for Electrophosphorescence with EQE of 32.2% and Low Efficiency Roll‐Off

AbstractThe integration of carbazole (Cz) unit and emissive iridium(III) complexes is a wise choice to facilitate hole injection/transport, charge balance, and achieve phosphorescent emitters for high‐performance organic light‐emitting diodes (OLEDs). Herein, three efficient Cz‐based Ir(III) complexes are synthesized and applied for OLEDs. The introduction of trifluoromethyl and trimethylsilyl units alter the photoelectric properties of these complexes, achieving photoluminescence (PL) from yellow (559 nm) to orange‐red (600 nm) with high PL quantum yields between 75% and 93% and short excited state lifetimes between 1.09 and 1.90 µs in doped thin films. Multilayer OLEDs using the three emitters exhibit highest current efficiency as high as 100.4 cd A−1 and maximum external quantum efficiency (EQEmax) up to 32.2% with very low efficiency roll‐off. Even at high brightness of 5000 and 10 000 cd m−2, the device D3 still can maintain the EQE at 28.2% and 27.9%, respectively. These results indicate that these complexes incorporating Cz units are potential substituents for efficient OLEDs.

Photoluminescence, photophysics, and photochemistry of the VB− defect in hexagonal boron nitride

Extensive photochemical and spectroscopic properties of the $V_B^-$ defect in hexagonal boron nitride are calculated, concluding that the observed photoemission associated with recently observed optically-detected magnetic resonance is most likely of (1)3E" to (1)3A2' origin. Rapid intersystem crossing from the defect's triplet to singlet manifolds explains the observed short excited-state lifetime and very low quantum yield. New experimental results reveal smaller intrinsic spectral bandwidths than previously recognized, interpreted in terms spectral narrowing and zero-phonon-line shifting induced by the Jahn-Teller effect. Different types of computational methods are applied to map out the complex triplet and singlet defect manifolds, including the doubly ionised formulation of the equation-of-motion coupled-cluster theory that is designed to deal with the open-shell nature of defect states, and mixed quantum-mechanics/molecular-mechanics schemes enabling 5763-atom simulations. Two other energetically feasible spectral assignments from amongst the singlet and triplet manifolds are considered, but ruled out based on inappropriate photochemical properties.

Deep-blue organic light-emitting diodes based on a doublet d\u2013f transition cerium(III) complex with 100% exciton utilization efficiency

Compared to red and green organic light-emitting diodes (OLEDs), blue OLEDs are still the bottleneck due to the lack of efficient emitters with simultaneous high exciton utilization efficiency (EUE) and short excited-state lifetime. Different from the fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), and organic radical materials traditionally used in OLEDs, we demonstrate herein a new type of emitter, cerium(III) complex Ce-1 with spin-allowed and parity-allowed d–f transition of the centre Ce3+ ion. The compound exhibits a high EUE up to 100% in OLEDs and a short excited-state lifetime of 42 ns, which is considerably faster than that achieved in efficient phosphorescence and TADF emitters. The optimized OLEDs show an average maximum external quantum efficiency (EQE) of 12.4% and Commission Internationale de L’Eclairage (CIE) coordinates of (0.146, 0.078).

Efficient rare earth cerium(III) complex with nanosecond d-f emission for blue organic light-emitting diodes.

In the field of RGB diodes, development of a blue organic light-emitting diode (OLED) is a challenge because of the lack of an emitter which simultaneously has a short excited state lifetime and a high theoretical external quantum efficiency (EQE). We demonstrate herein a blue emissive rare earth cerium(III) complex Ce-2 showing a high photoluminescence quantum yield of 95% and a short excited state lifetime of 52.0 ns in doped film, which is considerably faster than that achieved in typical efficient phosphorescence or thermally activated delayed fluorescence emitters (typical lifetimes >1 μs). The corresponding OLED shows a maximum EQE up to 20.8% and a still high EQE of 18.2% at 1000 cd m−2, as well as an operation lifetime 70 times longer than that of a classic phosphorescence OLED. The excellent performance indicates that cerium(III) complex could be a candidate for efficient and stable blue OLEDs because of its spin- and parity-allowed d−f transition from the Ce3+ ion.

Unique Role of Heterole-Fused Structures in Aromaticity and Physicochemical Properties of 7,8-Dehydropurpurins.

Porphyrins with a fused five-membered ring, such as 7,8-dehydropurpurins, have appeared as an emerging class of unique porphyrinoids. Their altered absorption spectra, relatively short lifetimes of excited states, and small HOMO-LUMO gaps arise from the harmony of the antiaromatic 20π-circuit and the aromatic 18π-circuit. In this regard, the electronic properties of 7,8-dehydropurpurins are expected to be controlled by modulating the contribution of the antiaromatic π-circuit to the whole aromaticity. Here the comparison of pyrrole- and phosphole-fused 7,8-dehydropurpurins is reported in terms of their aromaticity and physicochemical properties. The spectroscopic investigation revealed the larger contribution of the antiaromatic 24π-circuit in pyrrole-fused 7,8-dehyrdopurpurins than in phosphole-fused 7,8-dehydropurpurins. The DFT calculations also supported the feasibility of tuning the aromaticity of 7,8-dehydropurpurins by heterole-fused structures. Thus, the introduction of heterole-fused structures into porphyrinoids is a universal strategy to get new insight into aromaticity and their intrinsic properties in cyclic π-conjugated molecules.

Highly-efficient solution-processed deep-red organic light-emitting diodes based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand

Abstract Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol(L1), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L2), and 4-(4-methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L3) have been designed, synthesized, and characterized. All the compounds emit deep red emission with λmax values in the spectral range of 602–620 nm, high quantum yield 0.44 to 0.52 and short excited state lifetime τ (0.51–0.55 μs) due to dominant strong field ligands, resulting an efficient triplet metal-ligand charge transfer (3MLCT) excited state. Time dependent density functional theory (TD-DFT) calculations and electrochemical measurements of the compounds strongly support their genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly distinct electron density distributions of the LUMO and HOMO. The compounds show good thermal stability and quantum yield, these characteristics making them an ideal candidate to exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs were developed by using Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) in solution process as deep red emitters and device composed of Ir(piq)2(L3) exhibited an excellent external quantum efficiency of 14.9% and current efficiency of 10.8 cd/A with the stable CIE coordinates of (0.67,0.33).

Identification of the loss mechanisms in TiO2 and ZnO solar cells based on blue, piperidinyl-substituted, mono-anhydride perylene dyes

While perylene dyes have suitable properties for dye-sensitized solar cells (DSSCs), the performance has been limited by various factors that are not fully understood yet. We prepared DSSCs based on mesoporous TiO2 and ZnO with three blue piperidinyl-substituted perylene derivatives and three different electrolyte solutions. The perylene dyes have different non-conjugated side units and HOMO levels, and the anchoring group is a mono-anhydride moiety that would provide better compatibility with ZnO. Using current - voltage characteristics, incident and absorbed photon-to-electron conversion efficiency measurements (IPCE/APCE), stepped light induced transient measurements, and transient absorption spectroscopy (TAS), the limiting processes governing the solar cell performance are analyzed. With TiO2, weak electronic coupling between the dye and TiO2 appears to limit the injection efficiency, while for ZnO, the coupling seems stronger resulting in higher injection yield and higher APCE. The HOMO levels and structural differences of the dyes have only a minor impact on the reduction kinetics of oxidized dyes. DFT calculations illustrate that the HOMO and LUMO of the dye are spatially not well separated. This probably leads to a short-excited state lifetime, resulting in the relatively low APCE values. The TA measurements of the cells with and without redox couple show similar decays, suggesting the oxidized dyes in the solar cells are reduced not only by redox couple but also by electrons from TiO2 and ZnO, further decreasing the conversion efficiency. These results suggest the design rules to be followed for the successful implementation of all-organic perylene-based dyes for DSSCs.

Propeller type dibenzofurocarbazole as a new rigid donor moiety for highly efficient and long living thermally activated delayed fluorescence emitters

A carbazole extended 15H-dibenzofuro[3,2-a:3′,2′-c]carbazole (dBFCz) was developed as a propeller type strong donor moiety of thermally activated delayed fluorescent (TADF) emitters. Two TADF emitters, 2-(15H-dibenzofuro[3,2-a:3′,2′-c]carbazol-15-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (dBFCzCNTrz) and 15-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-15H-dibenzofuro[3,2-a:3′,2′-c]carbazole (dBFCzTrz), were synthesized using the dBFCz donor and diphenyltriazine acceptor. The dBFCzCNTrz and dBFCzTrz emitters showed high external quantum efficiency of 27.5% and 22.6% with short excited state lifetime, respectively. In particular, the dBFCz donor was much better than a common carbazole donor and benzofurocarbazole donor in enhancing the external quantum efficiency and device lifetime, which proposed that the dBFCz worked well as a donor moiety for TADF emitters.

Bis-tridentate N-Heterocyclic Carbene Ru(II) Complexes are Promising New Agents for Photodynamic Therapy.

Ruthenium(II) complexes developed for photodynamic therapy (PDT) are almost exclusively tris-bidentate systems with C2 or D3 symmetry. This is due to the fact that this structural framework commonly produces long-lived excited states, which, in turn, allow for the generation of large amounts of singlet oxygen (1O2) and other reactive oxygen species. Complexes containing tridentate ligands would be advantageous for biological applications as they are generally achiral (D2d or C2v symmetry), which eliminates the possibility of multiple isomers which could exhibit potentially different interactions with chiral biological entities. However, Ru(II) complexes containing tridentate ligands are rarely studied as candidates for photobiological applications, such as PDT, since they almost exclusively exhibit low quantum yields and very short excited-state lifetimes and, thus, are not capable of generating sufficient 1O2 or engaging in electron transfer reactions. Here, we report a proof-of-concept approach to make bis-tridentate Ru(II) complexes useful for PDT applications by altering their photophysical properties through the inclusion of N-heterocyclic carbene (NHC) ligands. Three NHC and two terpyridine ligands were studied to evaluate the effects of structural and photophysical modulations of bis-substituted Ru(II) complexes. The NHC complexes were found to have superior excited-state lifetimes, 1O2 production, and photocytotoxicity. To the best of our knowledge, these complexes are the most potent light-activated bis-tridentate complexes reported.